lektor plugins: please add lektor-markdown-header-anchors

Please add the plugin:

• https://www.getlektor.com/plugins/lektor-markdown-header-anchors/

i think this is important:

• on the tb-manual, to keep consistency with previous links i.e. secureconnections#managing_identities.
• makes the content cleaner, it will make a difference in a glossary page as it is seen at https://trac.torproject.org/projects/tor/wiki/doc/community/glossary#Torbutton